Flexible couplings for connecting adjoining shafts which may have their axes misaligned either through installation error or by design are well-known in the prior art and available for many applications. Most prior art flexible coupling designs, however, are limited in capability both as to durability and the degree of angular or axial misalignment permitted between the adjoining shafts (e.g., a drive shaft and load shaft). A few existing couplings utilize elastomeric material to accommodate misalignment, but they generally lack torsional stiffness and relative to their size can only transmit small amounts of torque. Conventional nonelastomeric types of flexible couplings, including the Cardan type of universal joint, permit a relatively high degree of shaft misalignment but do not provide a constant velocity relationship between the rotating shafts. Specialty mechanical couplings have been designed to provide such a constant velocity relationship, but these joints include bearings and a seal which must be lubricated and are subject to failure.
As an alternative to prior art couplings utilizing elastomeric material, and those universal joints such as the Cardan type, endless belt type couplings have been developed to enhance angular and axial misalignment accommodation capability of the coupling while providing sufficient torsional strength. Such couplings include a plurality of individual, flexible endless belt elements connected to one another and the connected members by bolts, pins or similar means. The endless belt elements are typically of equal length and form flexing structures consisting of a plurality of elements connected together in a variety of symmetrical patterns including generally square, polygonal or circular (e.g., octagonal, hexagonal) configurations. At least two flexing structures are joined together to form the link coupling, which in turn is connected at each end to the flange of the shafts to be joined. Disclosure of such couplings can be found in U.S. Pat. Nos. 4,790,794 and 4,377,386.
Endless belt type couplings provide numerous advantages over flexible type couplings including weight reduction, environmental resistance to corrosive elements, wear resistance and nonlubrication. Use of such couplings has been expanding to many applications. However, known endless belt type couplings are limited to use in applications where rotation is continuous in one direction. This limitation results from undesirable shrinkage and expansion the existing couplings undergo during torque reversals. Such shrinkage and expansion of the coupling is avoided during continuous unidirectional rotation of the coupling. Arrangement of the endless belts in the attachment of the drive and driven members results in continuous tension of the coupling during continuous unidirectional rotation regardless of direction. The belts are alternatively arranged into two groups, a first group of belts having a leading portion attached to the drive member and a trailing portion attached to the driven member, and a second group of belts each having a leading portion attached to the driven member and a trailing portion attached to the drive member. The terms leading and trailing are used as references of the ends of the belts with respect to rotation in one direction. For rotation in one direction, clockwise for example, torque is transferred from the drive to driven members by the "pull" of the belts placing each belt of the first group in tension. Belts of the second group are placed in minor compression but do not otherwise effect direct torque transfer between the members. Virtually all the torque transfer is carried by the belts of the first group, which are very soft in compression carry virtually none of the torque transfer between the members. This physical characteristic is normally described as the non-linearity load deflection characteristic of the individual belts. For rotation in the opposite direction the roles of the two groups of the endless belts are reversed.
The above-described arrangement of the known endless belt type coupling is satisfactory when rotation is continuous in either direction or when the rotation is slowly reduced to zero and then reversed on an intermittent basis. However, such couplings have undesirable dynamic characteristics that result in damage to the coupling and premature failure if the coupling is subjected to sudden rotation reversals. As shown in FIG. 6A, the known endless belt type couplings have a distinct non-linearity or a flat spot in their torque-deflection curves at zero torque. The non-linearity illustrates the shrinkage or contraction of the coupling from the compression mode, much like an ordinary rubber band upon release from stretching. Sudden rotation or torque reversals of the coupling results in "snapping" of the coupling between opposite tension modes. Furthermore, such couplings have been found to be laterally soft, resulting in an initial deflection of the center of axis of the coupling from the axis of rotation when couplings are used in conjunction with horizontal shafts. Such deflection results in an out of balance condition on the coupling upon rotation. Known endless belt couplings fail when subjected to sudden reversals as described above. This problem as shown in the non-linearity in their torque-deflection curves requires solution before such endless type couplings can be used in numerous reversible coupling applications.
Numerous applications of reversible endless belt type couplings exist. One such application is in cooling towers having fans that operate at high speeds and high torque in either rotary direction. Cooling towers are used in power plants, industrial plants and ships to continuously cool large quantities of water. Such water is generally chlorine treated to reduce bacteria growth. Requirements of couplings in such environments include sudden reversal rotation in either direction, misalignment up to about 5.degree., operating speeds up to about 7000 rpm and a continuous torque capacity of about 15,000 to about 40,000 in/lb with intermittent loads up to about 80,000 in/lb. Furthermore it is desirous the coupling withstand the environmental effects of chlorinated water.
One significant problem of cooling tower drive systems is the changing amount of misalignment. Cooling tower structures are generally of wood construction that can dimensionally change due to changing climatic conditions. Such structural changes result in variance in the misalignment of the drive system up to about 5.degree.. Another problem results from the sudden reversal of the direction of rotation of the drive shaft. Air flow in cooling towers is reversed for such purposes as melting ice from the structure. Often times the direction of rotation is suddenly reversed resulting in peak torque loads on the coupling.
One type of flexible coupling currently used in cooling tower drive systems is a stainless steel metal diaphragm coupling. However, such couplings are limited to maximum misalignment up to 1.degree. and lack environmental resistance to chlorinated water. Replacement of such couplings is generally required after a short period of continuous service. The use of such a coupling to solve the aforementioned cooling tower drive system problems has not proved satisfactory because of the foregoing shortcomings.
It is important that a suitable coupling for a cooling tower drive system be compact, lightweight, easy to maintain and capable of withstanding rotation in any direction. Such a coupling must also have a predictable longer service life, operate satisfactorily without requiring lubrication and operate under misalignment up to 5.degree..
While there are numerous available flexible couplings and endless belt type couplings suitable for unidirectional rotation, there is not currently available an endless belt type reversible coupling capable of meeting the aforedescribed conditions imposed in connecting a cooling tower fan to a drive motor in a manner permitting the cooling tower fan to rotate about an axis inclined at an angle up to about 5.degree. with respect to the drive motor shaft while permitting transfer of high speed and high torque between the motor and the fan.